[PATCH 4/4] block: Use blk_make_empty() after commits

[Max Reitz <mreitz@redhat.com>]

bdrv_commit() already has a BlockBackend pointing to the BDS that we
want to empty, it just has the wrong permissions.

qemu-img commit has no BlockBackend pointing to the old backing file
yet, but introducing one is simple.

After this commit, bdrv_make_empty() is the only remaining caller of
BlockDriver.bdrv_make_empty().

Signed-off-by: Max Reitz <mreitz@redhat.com>
---
 block/commit.c |  8 +++++++-
 qemu-img.c     | 19 ++++++++++++++-----
 2 files changed, 21 insertions(+), 6 deletions(-)

diff --git a/block/commit.c b/block/commit.c
index 8e672799af..24720ba67d 100644
--- a/block/commit.c
+++ b/block/commit.c
@@ -493,10 +493,16 @@ int bdrv_commit(BlockDriverState *bs)
     }
 
     if (drv->bdrv_make_empty) {
-        ret = drv->bdrv_make_empty(bs);
+        ret = blk_set_perm(src, BLK_PERM_WRITE, BLK_PERM_ALL, NULL);
         if (ret < 0) {
             goto ro_cleanup;
         }
+
+        ret = blk_make_empty(src, NULL);
+        if (ret < 0) {
+            goto ro_cleanup;
+        }
+
         blk_flush(src);
     }
 
diff --git a/qemu-img.c b/qemu-img.c
index 821cbf610e..a5e8659867 100644
--- a/qemu-img.c
+++ b/qemu-img.c
@@ -1065,11 +1065,20 @@ static int img_commit(int argc, char **argv)
         goto unref_backing;
     }
 
-    if (!drop && bs->drv->bdrv_make_empty) {
-        ret = bs->drv->bdrv_make_empty(bs);
-        if (ret) {
-            error_setg_errno(&local_err, -ret, "Could not empty %s",
-                             filename);
+    if (!drop) {
+        BlockBackend *old_backing_blk;
+
+        old_backing_blk = blk_new_with_bs(bs, BLK_PERM_WRITE, BLK_PERM_ALL,
+                                          &local_err);
+        if (!old_backing_blk) {
+            goto unref_backing;
+        }
+        ret = blk_make_empty(old_backing_blk, &local_err);
+        blk_unref(old_backing_blk);
+        if (ret == -ENOTSUP) {
+            error_free(local_err);
+            local_err = NULL;
+        } else if (ret < 0) {
             goto unref_backing;
         }
     }
-- 
2.25.4